| Computer Code: |
WRAN |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Guadalupian Epoch (PUG)
— Lopingian Epoch (PUL) |
| Lithological Description: |
The Werraanhydrit Formation has not generally been cored in the Southern North Sea and lithological information is therefore based largely on drill cuttings and wireline log evidence.
Shelf/slope sections around the basin margin are characterized by thick anhydrite with subordinate interbedded dolomite and halite. The anhydrite is generally described as white or yellowish white to pale grey, soft to hard, amorphous or massive and microcrystalline with sporadic argillaceous beds. It occurs as nodules, enterolithic bands, large cumulous masses and replacement crystals associated with microcrystalline dolomite (Taylor and Colter, 1975). The interbedded dolomite is commonly described as pale grey to pale brown, dense, microcrystalline, argillaceous and anhydritic. Irregular, dark laminae are present in the dolomite, which in places has a vaguely pelletoid fabric (Taylor and Colter, 1975).
The thin basinal facies commonly consists of a vertical sequence of four anhydrite/carbonate couplets which are recognised from both core examination and wireline log responses (Taylor and Colter, 1975; Taylor, 1980). Broadly, each unit comprises a l - 2 m layer of nodular and mosaic, displacive anhydrite that grades up into 4-6m of finely and evenly interlaminated microcrystalline anhydrite and dark, bituminous dolomite or limestone with some bands of anhydritic nodules (Taylor, 1980). |
| Definition of Lower Boundary: |
In sections around the basin margin, the base of the Werraanhydrit Formation is marked by a downward change from thick, white, microcrystalline, nodular and mosaic anhydrite with subordinate carbonate and argillaceous layers to pale to dark grey and brown, microcrystalline, calcitic dolomite and dolomitic limestone with anhydrite nodules of the Zechsteinkalk Formation. It is marked on wireline logs by a downward increase in gamma-ray values (e.g. 48/30-5 and 49/26-4).
In basinal sections, the base of the Werraanhydrit Formation is marked by a downward change from finely interlaminated anhydrite and dark, bituminous dolomite or limestone with basal layers of nodular and mosaic anhydrite, to dark, pyritic, argillaceous dolomite and limestone of the Zechsteinkalk Formation. Typically, it is marked on wireline logs by a downward increase in gamma-ray values (e.g. 44/21-1). Locally, the Zechsteinkalk Formation is absent and the Werraanhydrit Formation rests disconformably upon organicrich mudstone of the Kupferschiefer Formation (e.g. 38/24-1) or unconformably on pre-Zechstein strata (e.g. 37/23-1, Upper Old Red Group). |
| Definition of Upper Boundary: |
Around the basin margin, the top of the Werraanhydrit Formation is normally defined by a downward change from pale grey to pale brown oolitic, pelletoidal, bioclastic and pisolitic, dolomitic grainstones and carbonate mudstones of the Hauptdolomit Formation, to nodular and mosaic, white anhydrite or anhydrite with carbonate layers. Typically, it is marked on wireline logs by a downward decrease in gamma-ray values and locally by a decrease in velocity. In basinal sections, the top of the Werraanhydrit Formation is taken at a gradual downward change from dark brown to black, bituminous, thinly laminated carbonates of the Hauptdolomit Formation ('Stinkschiefer fades') to finely interlaminated anhydrite and dark, bituminous dolomite or limestone. It corresponds to a downward decrease in gamma-ray values, immediately below a high gamma spike (e.g. 44/21-1). |
| Thickness: |
At the southern margin of the basin, and bordering the Mid North Sea High to the north (e.g. 36/26-1), the Werraanhydrit Formation forms a prograding shelf/slope wedge about 50 km wide that abuts, and is enclosed by, the shelf facies of the Zechsteinkalk Formation. This shelf/slope facies is typically 100-200 m thick, but locally reaches up to 300 m. The Werraanhydrit Formation thins rapidly basinwards, and across most of the basin floor is about 10-25 m thick. |
| Geographical Limits: |
The Werraanhydrit Formation is present throughout most of the Southern North Sea Basin. It is, however, absent through non-deposition from the London-Brabant Platform which was probably an area of palaeorelief. It is also absent from the southern margins of the basin where the shelf facies of the Zechsteinkalk Formation is directly overlain by the Basalanhydrit Formation and red mudstones and sandstones of the Blakeney Formation. It is not recognized in the north of quadrants 37, 38 and 39, on the Mid North Sea High, where equivalent strata are included in the Halibut Carbonate Formation (Cameron, 1993). |
| Parent Unit: |
Zechstein Group (ZG)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Reference Section |
North Sea well 36/26- 1: 1419-1472 m (4655-4830 ft) (Johnson et al., 1994). |
| Reference Section |
North Sea well 44/21- 1: 3834.5-3855 m (12580-12648 ft) (Johnson et al., 1994). |
| Reference Section |
North Sea well 49/26- 4 (Rhys, 1974, table 2, fig. 3) 1855-2007 m (6086-6584 ft) (revised upper boundary depth) (Johnson et al., 1994). |
| Reference Section |
North Sea well 48/30- 4: 1890-2065.5 m (6200-6777 ft) (Johnson et al., 1994). |
| Reference(s): |
| Cameron, T D J. 1993. 4. Triassic, Permian and pre-Permian of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| NAM and RGD (Nederlandse Ardolie Maatschappij and Rijks Geologische Dienst) 1980. Stratigraphic nomenclature of the Netherlands. Transactions of the Royal Dutch Geological and Mining Society of Delft. |
| Rhys, G H. 1974. A proposed standard lithostratigraphic nomenclature for the southern North Sea and an outline structural nomenclature for the whole of the (UK) North Sea. Report of the Institute of Geological Sciences, 74/8. |
| Smith, D B. 1980. The evolution of the English Zechstein basin. In: Fuchtbauer, H, and Peryt, T M (eds.) The Zechstein basin with emphasis on carbonate sequences. Contributions to Sedimentology 9, 7-34. |
| Smith, D B, Brunstrom, R G W, Manning, D I, Simpson, S and Shotton, F W. 1974. A correlation of Permian rocks in the British Isles. Geological Society of London Special Report No. 5. |
| Smith, D B, Harwood, G M, Pattison, J and Pettigrew, T H. 1986. A revised nomenclature for Upper Permian strata in eastern England. 9-17 in Harwood, G M and Smith, D B (editors). The English Zechstein and Related Topics. Geological Society Special Publication No.22. |
| Taylor, J C M. 1980. Origin of the Werraanhydrit in the UK Southern North Sea- a reappraisal. In: Fuchtbauer, H, and Peryt, T M (eds.) The Zechstein basin with emphasis on carbonate sequences. Contributions to Sedimentology 9, 91-113. |
| Taylor, J C M. 1990. Upper Permian-Zechstein. In: Glennie, K W (ed.) Introduction to the petroleum geology of the North Sea, 153-190. Blackwell Scientific Publications, Oxford. |
| Taylor, J C M, and Coulter, V S. 1975. Zechstein of the English sector of the Southern North Sea. In: Woodland, A W (ed.) Petroleum and the continental shelf of North-West Europe, 249-263. Applied Science Publishers, London. |
| Richter-Bernburg, G. 1986. Zechstein 1 and 2 Anhydrites: facts and problems of sedimentation. In: Harwood, G M, and Smith, D B (eds.) The English Zechstein and related topics. Geological Society, London, Special Publication No. 22, 157-163. |
| Taylor, J C M, and Fong, G. 1969. Correlation of Upper Permian strata in east Yorkshire and Durham. Nature, London, 224, 173-175. |
| Johnson, H, Warrington, G and Stoker, S J. 1994. 6. Permian and Triassic of the Southern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
